1. Technical Field
The present invention relates to an organic electroluminescent unit (hereinafter referred to as an organic EL unit), a method for manufacturing the organic EL unit, and an electronic apparatus.
2. Related Art
Organic EL devices, that is, organic light-emitting diodes (OLEDs) are attracting much attention because they can be used as light sources for thin, lightweight displays. Full-color displays including the organic EL devices have the following advantages: (1) high color purity, (2) no viewing-angle dependence, and (3) low power consumption. However, the full-color displays have a disadvantage that their lifetimes are short.
There are various possible causes for the fact that the organic EL devices have a short lifetime. For example, light-emitting layers are deteriorated by oxygen or moisture. It is known that the surfaces of the light-emitting layers are degraded during the period from the completion of the light-emitting layers to the formation of electron transport layers. JP-A-2004-335475 (hereinafter referred to as Patent Document 1) discloses a technique in which an emission-protecting layer is formed immediately after the completion of the light-emitting layer so as to protect the surface of the light-emitting layer from contamination.
There are other causes for the fact that the organic EL devices have a short lifetime. The inventors have found that the following devices have different lifetimes: an organic EL device including a light-emitting layer formed within a short time after the completion of a hole transport layer and an organic EL device including a light-emitting layer formed within a relatively long time (for example, about 30 minutes) after the completion of a hole transport layer. Although the reason for the difference therebetween is not clear, the inventors assume the reason for the difference therebetween to be that in the latter one, the surface of the hole transport layer is degraded during the period from the completion of the hole transport layer to the formation of the light-emitting layer, which is degraded because of the degradation of the hole transport layer. Although hole transport layers and light-emitting layers are formed in a vacuum if being formed by, for example, vapor deposition, the hole transport layers may be degraded by a minute amount of oxygen or moisture after the completion of the light-emitting layers. According to the technique disclosed in Patent Document 1, the emission-protecting layer protects the light-emitting layer; however, this technique is ineffective in preventing or suppressing the hole transport layer from being degraded and the light-emitting layer from being degraded due to the degradation of the hole transport layer.
It is known that a light-emitting layer made of a low-molecular-weight material contains a host material and a guest material (a dopant) and holes and electrons recombine with each other in molecules of the dopant to emit light. Since the host material and the dopant are mixed together by co-deposition, it takes a long time to stabilize the deposition rates of these materials. Therefore, this light-emitting layer cannot be necessarily formed immediately after the completion of a hole transport layer.